Method of loading a stent into a sheath

ABSTRACT

There is disclosed herein a method of loading a self-expanding stent ( 10 ) into a delivery sheath ( 12 ) that is by itself strong enough to preserve the stent in a radially compressed pre-deployment configuration, comprising the steps of: (i) compressing the stent ( 10 ) to a relatively small diameter; (ii) providing a loading sheath ( 18 ) with an inner diameter d 1 ; (iii) with the stent ( 10 ) exhibiting an outside diameter of d 2 , where d 2 &lt;d 1 , providing relative translational movement between the stent ( 10 ) and the loading sheath ( 18 ), whereby the stent is accommodated within the lumen of the loading sheath ( 18 ); (iv) advancing the loading sheath ( 18 ), containing the stent ( 10 ), into the lumen ( 16 ) of the delivery sheath ( 12 ); and {v} deploying the stent into the lumen ( 16 ) of the delivery sheath ( 12 ), by withdrawing the loading sheath ( 18 ) while restraining the stent ( 10 ) from axial movement out of the lumen ( 16 ) of the delivery sheath ( 12 ).

FIELD OF THE INVENTION

This invention relates to a method of loading a self-expanding stent into a delivery sheath that is part of a catheter for transluminal delivery of the stent to a stenting location in the body of a patient, with the stent being deployed at that location by removal from the stent of the constraint on its radial expansion that is provided up to that point by the sheath.

Stent designers are succeeding in creating ever more sophisticated stents and stent grafts, for location in ever more sites in the body. There is consequently pressure on designers of stent delivery systems, to come up with designs for catheters that will reach ever more demanding requirements to convey such stents to the desired locations in the body.

The usual way to express a stent diameter is in units of “French”. One French is one third of a millimetre. Thus, a stent delivery system with a passing diameter of “6 French” has an outside diameter of 2 mm. For a manufacturer of stents and stent delivery systems, a reduction of diameter from, say, 6 French to 5 French is a powerful technical advantage, and a great challenge for designers of stents and stent delivery systems. Measures that are capable of delivering even the tiniest reduction of the passing diameter of a stent delivery catheter might still be of considerable interest to such designers, provided the catheter is still as flexible, pushable and capable of reliable operations as a fatter catheter.

THE PRIOR ART

One way to release a stent from the constraint of a sheath is to run a slitter down the length of the sheath that surrounds the stent. See, for example, WO 01/08599, WO 93/17636 and WO 98/20812. However, a far more commonly used method is to pull the sheath proximally, relative to the stent, to release the stent progressively, starting at its distal end. See, for example, U.S. Pat. No. 5,833,694 Poncet. However, pulling on the sheath from its proximal end, will impose tensile stresses on the sheath material, in the length direction of the sheath. The sheath must therefore be physically strong enough, not only to resist the radially outward spring force that is coming from the self-expanding stent constrained within the sheath, but also the stresses to which the sheath is subject during the time that it is being pulled proximally, to release the stent. A force that tends to stretch a tube will also tend to reduce the diameter of that tube. A simple example is the toy called the “Chinese finger” that is in the form of a braided tube. The child slips it over his finger and then pulls the free end of the tube, causing the braiding to neck down, and grip tightly the finger within the lumen of the braided tube. Intuitively, one can appreciate that pulling on a sheath from the proximal end, when there is a self-expanding stent inside the lumen of the sheath, is a situation fraught with the possibility of unwanted additional, friction-generated loads that might be an impediment to smooth release of the stent.

One can appreciate that the wall thickness of the sheath makes a contribution to the totality of the passing profile of the stent delivery system, at least at the distal end of the catheter delivery system, in the “capsule” that contains the stent to be delivered by that system, for any such capsule is liable to be that portion of the length of the catheter that exhibits the greatest passing diameter. It is normally possible to devise a catheter delivery system in which the length of the catheter, the shaft, between the proximal end of the catheter held in the hand of the operator, and the distal end that exhibits the capsule containing the stent, will be of smaller passing diameter. Thus, designers are interested to reduce the wall thickness of the sheath surrounding the self-expanding stent, to the extent that is possible, consistent with safe operation of the system. It is of course intolerable for the sheath component to fail during deployment of the stent. The sheath component must be robust enough to withstand less than optimal handling of the stent delivery system, by medical practitioners, during placement of the stent.

It is an object of the present invention to advance stent delivery system design to achieve a smaller passing profile, consistent with simplicity of design and reliability of operation.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention there is provided a method of loading a self-expanding stent into a delivery sheath that is by itself strong enough to preserve the stent in a radially compressed pre-deployment configuration, comprising the steps of:

-   -   i) compressing the stent to a relatively small diameter     -   ii) providing a loading sheath with an inner diameter d1     -   iii) with the stent exhibiting an outside diameter of d2, where         d2<d1, providing relative translational movement between the         stent and the sheath, whereby the stent is accommodated within         the lumen of the loading sheath;     -   iv) advancing the loading sheath, containing the stent, into the         lumen of the delivery sheath; and     -   v) deploying the stent into the lumen of the delivery sheath, by         withdrawing the loading sheath while restraining the stent from         axial movement out of the lumen of the delivery sheath.

In accordance with another aspect of the present invention, there is provided apparatus for installing a self-expanding stent in a sheath of a stent delivery catheter, the apparatus comprising a loading sheath with a lumen for containing the self-expanding stent, the apparatus deploying the stent into the delivery sheath by advancing into the lumen of the delivery sheath the loading sheath containing the stent, and then withdrawing from the lumen of the delivery sheath the loading sheath but not the stent.

According to a further aspect of the present invention there is provided a self-expanding stent (or stent graft) within the lumen of a delivery sheath of a stent delivery system, the delivery sheath being formed of polymeric material that has been cold drawn with the stent in situ, within the lumen of the sheath.

In a preferred embodiment of the method of the invention, the delivery sheath is subject to a lengthwise strain, after the stent has been installed in the lumen of the sheath, to draw down the inner diameter of the delivery sheath to a diameter d3, which is less than diameter d1.

In one preferred embodiment, the sheath is one which can be parted along a line extending axially the full length of the stent, to allow the stent to push the material of the longitudinally parted sheath of the way, during the radial expansion of the stent that occurs as the stent deploys into a bodily lumen. Conveniently, the sheath is made of polyethyleneterephthalate (PET) and typically has a wall thickness of 30 μm prior to any drawing down of the diameter after placement of the stent in the lumen of the sheath.

It may be useful to design into the sheath a specific zone for initiation of the cold drawing strain, so that such strain spreads outwardly, progressively, along the length of the sheath, from the strain initiation zone, upon imposition on the sheath of the longitudinal stress needed to initiate the strain. Generally, it will be convenient to locate any such strain initiation zone somewhere between the opposite ends of the stent, such as midway along the length of the stent. The form of any such strain initiation zone could be a portion of the sheath that exhibits a reduced wall thickness.

Medical practitioners are seeking ever longer and more flexible stents. Such stents are correspondingly more problematic to handle, in such a way that no portion of the stent matrix is ever subject to a level of stress that prejudices the fatigue performance of the stent. Whereas short and robust stents tolerate being pushed from one end, long and flexible delicate stent designs are vulnerable to over-stressing, if not supported along their full length during loading into a sheath and during deployment out of that sheath. One way to provide support for a stent graft along the full length of the lumen of that graft can be found in the disclosure of Applicant's WO 2004/096091. Thus, for loading a stent or stent graft into the loading sheath of the present invention, it will generally be desirable to employ a tool that extends along the lumen of the stent and supports the stent matrix from within that lumen. Once the stent has been installed in the delivery sheath, designers have the option of leaving the tool in place, within the lumen of the stent, for making a contribution to the construction and operation of the delivery system for the stent or, in the alternative, removing the tool from the lumen of the stent, once the stent has been installed in the delivery sheath, and using for stent support during deployment of the stent some other component within the construction of the delivery system. Thus, an optional feature of the method of the present invention is the step of introducing into the lumen of the stent, as part of the process of installing the stent in the delivery sheath, a component of the delivery system that will deliver the stent to the stenting site within the body.

As for strategies for releasing the stent from the sheath, when the stent is deployed in the body at a stenting site, see Annex A that follows at the end, and forms part, of this description.

As to the drawing down step, it is to be remembered that the sheath is of remarkably small wall thickness and so amenable to being cold-drawn in a simple manual operation in which the sheath is gripped in the hands of a skilled person. The left and right hands can simply be pulled apart from each other, simply to stretch the sheath in the portion of its length surrounding the stent. However, such procedures are not ideally suited to meet rigorous quality control procedures. It is therefore envisaged that the manually-achieved cold drawing step will, in real production, be replaced by a rigorously specified mechanical step so that the stretching process is performed always in the same way, for every delivery system produced. One envisages machine components that constitute first and second sheath grippers, one at each end of the sheath, beyond the respective end of the stent within the sheath, such grippers likely gripping the sheath all around the circumference of the sheath, probably with a high enough friction grip to hold the sheath ends securely, possibly with grip surfaces of silicone rubber material, with the first and second sheath grippers moving apart from each other in a controlled fashion, with sensors to provide control engineering feedback so that the sheath is always cold-drawn in the same manner.

Sheath delivery systems with a distal end capsule having an outside diameter of 6 French, or even less, are well within the contemplation of the inventors of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show more clearly how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

FIG. 1 is a longitudinal axial section through a device for loading a stent into a delivery sheath;

FIG. 2 shows the device of FIG. 1, but with the stent partially loaded in the sheath; and

FIG. 3 is a flow diagram of method steps for installing a stent or stent graft within a sheath, in accordance with the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a crimped covered stent 10 ready for deploying from a loading sheath into a delivery sheath 12, by advancing the stent 10, in a loading tool 14 into the lumen 16 of the sheath 12, in the direction of arrow F.

The stent 10 is a radially self-expanding nickel titanium alloy stent covered in an ePTFE film. This covered stent is constrained by a loading sheath 18 in a radially compact disposition. The stent is installed in the loading sheath 18 by a “crimping” procedure known per se in which a jig (not shown) compresses the stent radially inwardly, down to its compact disposition, whereupon the stent 10 and the loading sheath 18 are moved relative to each other axially (usually by holding the sheath 18 stationary and translating the stent 10 into the lumen of the sheath 18).

Before the stent is crimped, there is inserted in its lumen a shaft 24 carrying a sequence of rings standing proud of the cylindrical surface of the shaft, or a spiral thread 22 running along the shaft. The covered stent is crimped down onto the shaft, whereupon the shaft can push the stent along its axis, supporting the stent all the way along the length of the stent, within its lumen. For more details, see Annex A, at the end of this description.

In accordance with the present invention, the stent 10 is then “deployed” into the lumen 16 of the delivery sheath 12. For that deployment, the confining sheath 18 is pulled proximally by a pulling tube 26 contiguous with the proximal end 28 of the loading sheath 18.

During this pulling, one restrains the shaft 24 from axial movement, which prevents the stent from moving axially with the retreating loading sheath 18, so that the sheath remains in the desired location, inside the delivery sheath 12. The delivery sheath 12 is of rather thin-walled PET tubular material. It is gripped at its proximal end 30 by an annular gripper chuck 32 and gripped at its distal end 34 by a similar annular gripping chuck 36, so that the two chucks 32 and 36 hold the opposite ends 30 and 34 of the delivery sheath 12 at a desired distance apart, while the stent is deployed into the sheath lumen 16.

This deployment process can be seen, partially completed, in FIG. 2 of the drawings, where like reference numerals identify the same components as are shown in FIG. 1. It will be appreciated that the loading sheath 18 has moved proximally, in the direction of arrow f, relative to the stent and to the stent pusher annulus 22, thereby releasing the stent 10 into the lumen 16 of the delivery sheath 12 progressively, starting at the distal end 40 of the stent, through a radially expanding portion 42 of the stent that travels the full length of the stent, from the distal end 40 to the proximal end 44 of the stent, not yet free of the confining loading sheath 18 as shown in FIG. 2. Once the full length of the stent 10 is clear of the loading sheath 18, the full radially outwardly directed stenting force from the stent 10 is carried by the delivery sheath 12, and the lumen 46 of the stent is big enough for simple proximal withdrawal from that lumen of the tubular element 24 of the loading machine.

As explained above, the stent 10 is now radially confined within the delivery sheath 12 and can be regarded as a precursor for a “capsule” for placement at the distal end of a catheter delivery system for the stent.

The process is presented in a block diagram, in drawing FIG. 3. However, FIG. 3 identifies further steps of the manufacturing process. The sequence of steps in FIG. 3 is identified by references A to I and we have the following comments on these process steps.

Step A is the step of providing the stent or stent graft of choice. A bare stent is one without any covering. A stent graft is a covered bare stent and the covering is typically of expanded polytetrafluoroethylene (ePTFE) but other covering materials are known to those skilled in the art.

Step B is the step of installing in the lumen of the chosen stent an inner catheter tool and the one preferred by the present Applicant is that described in its earlier publication WO 2004/096091 which goes by the acronym “ALFER”.

Step C is the step of crimping the stent to bring the outside diameter of the chosen stent down to one that is small enough for loading the crimped stent into the chosen loading sheath 18. Next, step D, the crimped stent is translated axially into the lumen of the loading sheath 18. Then, the loading sheath carrying the stent is translated (as explained in FIGS. 1 and 2) into the lumen 16 of the delivery sheath 12 and the loading sheath 18 is withdrawn in order to deploy the stent into the delivery sheath. As noted on FIG. 3, the delivery sheath 12 is conveniently a PET tube having a length about three times that of the stent, with a pre-thinned zone in the PET tube located midway along the length of the stent in the lumen of the PET tube.

Once the stent is deployed, the inner catheter loading tool can be removed, step F, from the lumen of the deployed stent. At this point, step G, any desired component of the catheter delivery system which is to be located in the lumen of the stent can now be introduced into that lumen.

Having prepared the stent lumen, the delivery sheath can now be cold-drawn, lengthwise, to impose substantial strain on the material of the delivery sheath. This strain will have the effect of reducing the wall thickness of the delivery sheath, and generating hoop stresses within the delivery sheath being cold-drawn, which hoop stresses will tend to compress the stent inside the lumen of the delivery sheath, and therefore marginally reduce its diameter. The Fig. block diagram carries notes in relation to this step H of the process. These notes teach to use a soft sticky material to grip the delivery sheath for achieving the cold-drawing strain. That strain is conveniently imposed on the sheath stepwise, commencing at the midpoint of the length of the sheath and repeating the cold-drawing process, stepwise, until a final desired outside diameter profile is achieved for the sheathed stent. Thus, in the final step I of the process, the stent-bearing capsule is ready for combining with other components of the catheter system that is to deliver the stent to the site of stenting in the body of a patient.

Applicant has an earlier-dated patent application, GB-0720327.6 which described a sheath splitting system. Annex A to this description follows immediately below, and provides the reader with an enabling description of one way to provide a splitting system for the sheath delivery catheter system described above.

Readers will understand that the illustrated embodiments are non-limiting. Readers have enough background technical knowledge to modify the specific teachings above so as to conform to their own perceptions of good design and conformity with their own stent products or other requirements of the market.

There now follows Annex A (comprising 29 sheets of text corresponding to the description and claims of Annex A, the associated drawings of which are incorporated as FIGS. 1A to 7A of this application), followed in turn by the claims in which the present invention is defined. Annex A forms part of and is incorporated into the present description, and its pages are correspondingly numbered in sequence with the present specification. 

1. A method of loading a self-expanding stent into a delivery sheath that is by itself strong enough to preserve the stent in a radially compressed pre-deployment configuration, comprising the steps of: i) compressing the stent to a relatively small diameter; ii) providing a loading sheath with an inner diameter d1; iii) with the stent exhibiting an outside diameter of d2, where d2<d1, providing relative translational movement between the stent and the loading sheath, whereby the stent is accommodated within the lumen of the loading sheath; advancing the loading sheath, containing the stent, into the lumen of the delivery sheath; and v) deploying the stent into the lumen of the delivery sheath, by withdrawing the loading sheath while restraining the stent from axial movement out of the lumen of the delivery sheath.
 2. Method according to claim 1, further comprising the step of, with the stent within the lumen of the delivery sheath, imposing lengthwise strain on the sheath, to draw down the inner diameter of the delivery sheath to a diameter d3 which is less than d1.
 3. Method according to claim 1, in which the delivery sheath is one which can be parted along a line extending axially the full length of the stent, to allow the stent to push the material of the longitudinally parted sheath out of the way, during the radial expansion of the stent that occurs as the stent deploys into a bodily lumen.
 4. Method according to claim 1, wherein the sheath is of PET.
 5. Method according to claim 2, wherein the sheath prior to said strain has a wall thickness of the order of 30 μm.
 6. Method according to claim 2, wherein the sheath exhibits a strain initiation zone from which the strain spreads out, progressively along the length of the sheath.
 7. Method according to claim 6, including the step of positioning the strain initiation zone between the opposing ends of the stent.
 8. Method according to claim 7, wherein the strain initiation zone is located mid-way between the stent ends.
 9. Method according to claim 6, wherein the strain initiation zone comprises a portion of the sheath that exhibits a reduced wall thickness.
 10. Method according to claim 1, in which a tool is used to advance the stent into the sheath, which tool has a portion that extends along at least a part of the length of the lumen of the stent.
 11. Method according to claim 10, in which said tool is removed from the stent, before the step of drawing down the sheath is completed.
 12. Method according to claim 1, claims, including a step of introducing into the lumen of the stent, before the step of drawing down the sheath is completed, a component of a catheter delivery system for said stent.
 13. Method according to claim 1, including a step of providing within the lumen of the sheath, prior to drawing down said sheath onto the stent, a splitter that lies between the stent and the drawn down sheath, which splitter is capable when actuated, of splitting the sheath to permit the stent to deploy.
 14. Method according to claim 2, wherein a tool is used to impose the strain on the sheath, the tool comprising first and second sheath grippers, one to grip each opposite end of the sheath, beyond the respective end of the stent inside the sheath.
 15. Method according to claim 14, wherein at least one of the sheath grippers grips the sheath all around the circumference of the sheath.
 16. Method according to claim 15, in which at least one of the grippers exhibits a high friction grip surface to contact the sheath, which grip surface is of a silicone rubber material.
 17. Method according to claim 1, in which the sheath drawn down over the stent exhibits an outside diameter of not more than 6 French (2 mm). 